In the storage or transmission of digital information, the bits or symbols of the user data are actually transmitted or stored via a physical medium or mechanism whose responses are essentially analog in nature. The analog write or transmit signal going into the storage/transmission medium or channel is typically modulated by channel bits (typically run-length limited or RLL bits) that are an encoded version of the original user-data bits (non-return-to-zero or NRZ bits). The analog read or receive signal coming from the medium is demodulated to detect or extract estimated channel bits, which are then decoded into estimated user-data bits. Ideally, the estimated user-data bits would be an identical copy of the original user-data bits. In practice, they can be corrupted by distortion, timing variations, noise and flaws in the medium and in the write/transmit and read/receive channels.
The process of demodulating the analog read signal into a stream of estimated user-data bits can be implemented digitally. Digital demodulation in magnetic mass storage systems requires that the analog read signal be sampled at a rate that is on the order of the channel-bit rate. Maximum-likelihood (ML) demodulation is a process of constructing a best estimate of the channel bits that were written based on digitized samples captured from the analog read signal.
FIG. 1 shows an exemplary read signal 100, which is a positive-going pulse generated by an inductive read head, for example, from a single medium transition such as transition 103 from North-South to South-North magnetization of track 104 on a rotating disk. Typically, the write signal modulates a transition in the state of the medium to write a channel bit of 1 and modulates the absence of a media transition to write a 0 channel bit. Thus, transition 103 corresponds to a single channel bit of value 1 in a stream of 0's.
It is common to use run-length-limited (RLL) encoding of the original user data bits, which are arbitrary or unconstrained, into an RLL-encoded stream of channel bits. It may be desirable that there be no less than d zeroes between ones; that is, that the medium transitions be spaced by at least d+1 channel bit times. This constraint can help keep to a manageable level the interference effects among the pulses in the analog read signal. On the other hand, because medium transitions provide timing information that must be extracted from the read signal to ensure synchronization of the demodulator with the pulses in the read signal, it may be desirable that there be no more than k zeroes between ones; that is, that there be a medium transition at least every k'th channel bit time. An RLL(d,k) code is a code that can encode an arbitrary stream of original user-data bits into a stream of channel bits such that the encoded channel bit stream satisfies these two constraints. An RLL code has a theoretical capacity which limits the number of user bits which can be represented in a given number of RLL bits. The capacity is a function of the d and k constraints with d=0 and k=infinite being the limiting (unconstrained) case with a capacity of exactly one. The capacity of an RLL (1,7) code for example is just slightly greater than 2/3 and is exactly 2/3 for any practical implementation, meaning that every pair of user bits will map to exactly three RLL bits.
In FIG. 1, sample set 101 shows the values of four samples in the case of side sampling of read signal 100; i.e. 0.333, 1.0, 1.0, and 0.333. Sample set 101 is equivalent to the set 1, 3, 3, 1; that is, only the ratios among samples are significant. A signal model gives rise to an expected sample sequence for a single or isolated transition in media state. Typically, only a few samples of an isolated media transition are non-zero; in this case, four are non-zero. In a side-sampled signal model such as 1, 3, 3, 1, timing circuitry in the demodulator attempts to maintain a lock on the incoming signal such that two adjacent samples on opposite sides of the peak of an isolated pulse have equal amplitudes and samples are taken at roughly equal time intervals, each a single channel bit time. Synchronization of the samples with the spacing of the bits written on the medium is maintained by a timing recovery loop which is in essence a phase-locked loop. Other sample timing arrangements may be useful. In center sampling, the timing circuitry tries to lock the sample times to the read signal pulses such that one sample occurs at the peak of each pulse. Sample set 102 shows the values of four samples in the case of center sampling of a similar read signal 104; i.e., 0.5, 1.0, 0.5, and 0.0 (or 1.0, 2.0, 1.0 and 0.0 depending on the arbitrary normalization used). An expected sample sequence of 1, 2, 1, 0 corresponds to the signal model known in the prior art as Extended Partial-Response Class IV (EPR4). Such sample sequences are samples of a continuous-time analog read-signal waveform such as may be produced in the readback circuitry of a magnetic storage device. For a system that is bandwidth limited to 1/(2T), where T is the sample spacing in time, the sampling theorem declares that the continuous time waveform must be superposition of sinc functions (sinc(x) is defined as sin(x)/x for x&lt;&gt;0, and as 1 for x=0), with one sinc function centered at each sample point and of amplitude equal to that sample value and with zero crossings at all other sample points. As an example, in saturation magnetic recording, the current in an inductive write head takes on values of +1 and -1. The basic excitation applied to the recording channel is a step in current from +1 to -1, vice versa, in the analog write signal. This step in write current produces a transition in the magnetization state of the medium as it moves past the head. When an inductive read head is passed over this magnetic medium transition, a voltage pulse is induced by the bandwidth limited differentiating interaction of the head with the magnetization of the medium. By suitable filtering or equalization, the sequence of samples on an isolated transition response pulse can be made to { . . . , 0, 0, 1, 2, 1, 0, 0, . . . }, in which case the recording or transmission channel matches the EPR4 signal model. Another sample sequence well known in the prior art is the Partial Response Class IV signal model (PR4), which corresponds to an expected sample sequence of 0, 1, 1, 0. Further, as one is designing or taking measurements on a write/media/read channel, it may be desirable to take into account the exact response, noise and distortion characteristics of the channel in selecting the signal model to be implemented in the demodulator. Thus, there is a need for a demodulator that is programmable as to the signal model, or expected sequence of sample values for an isolated medium transition. In situations such as mass information storage in magnetic media, significant storage-system speed and capacity gains can be realized if the information bits can be closer together in position/time on the medium. Further, as medium transitions are more closely positioned, the writing and reading processes become more sensitive to the distortion, timing variations and noise that are inevitably introduced in the processes of writing, storing, and reading. Also, as the transitions become closer, the ability of the medium to fully transition from, say, North-South magnetization to South-North magnetization may be taxed. Also, as the medium transitions become closer, interference effects increase among adjacent or nearby transitions. FIG. 2 shows how positive-going pulse 200 from first medium transition 201 combines with negative-going pulse 202 from second transition 203 to produce analog read signal 204, which can be viewed as the interference of the two pulses. Adjacent medium transitions always give rise to read pulses of opposite polarities because they always are created by transitions of opposite types, for example North-South changes to South-North in transition 201, so adjacent transition 202 must be South-North changing back to North-South. Read signal 204 might give rise to a sequence of samples such as 0.333, 1.0, 0.667, -0.667, -1.0, 0.333. To the extent that the read process is linear (and it may not be entirely linear), the voltage waveform induced in the read head will be the superposition of a sequence of pulses, where each pulse is the response to an isolated magnetic transition on the medium. Clearly, engineering a high-performance read channel is a complex challenge given the combined effects of the limited sampling rate in a digital demodulator, possibly incomplete transitions in the medium, interference among read signal responses to medium transitions, and distortion, timing variations, noise and flaws in the medium and in the write and read channels. The prior art uses a method known as partial-response signaling to increase medium transition rates. Partial-response signaling is described in the book "Digital Transmission of Information" by Richard E. Blahut, 1990, pp 139-158 and 249-255. This method allows the analog response of the storage/transmission medium and of the write/transmit and read/receive circuitry to a medium transition to overlap with the response to adjacent transitions associated with subsequent information bits. If properly implemented, this method can achieve higher information bit rates/densities than the alternative of requiring the medium transitions to be spaced such that the read signal responses do not overlap. Such a method requires a sequence detector which can make its decisions not on a bit-by-bit basis but by examining the context of the surrounding read signal.
In a magnetic disk drive, the surface of the magnetic media is logically divided into concentric rings called tracks. The distance around the track varies as a function of the radius at which the track lies. Since it is desirable to keep the rate of revolution of the disk constant to avoid mechanical delays in accelerating and decelerating the disk, it is necessary to either store an amount of data on each track which is proportional to the length of the track (this requires a different data transfer rate for each track) or to vary the physical transition spacing on the medium so that pulses are widely separated at the outside diameter and crowded very close at the inner diameter of the recording surface (this is wasteful of the magnetic medium which is only sparsely used at the outer diameter). A practice known as zoned recording is a popular compromise between these two extremes. In zoned recording, a group of tracks (a zone) is established in which every track in the zone holds the same amount of data. Thus each zone requires a different data transfer rate, but the number of data transfer rates which need be supported is reduced (more coarsely quantized). This still leaves a variation in the physical spacing of transitions between the inside and outside diameters of each zone resulting in a variation in pulse shape.
Partial-response signaling has just recently been incorporated into mass storage devices in, a limited form. One prior-art magnetic disk drive using partial-response signaling only supports PR4 (pulses with the samples of . . . , 0, 1, 1, 0, . . . ) . PR4 signaling has only very limited inter-symbol interference evidenced by only two non-zero samples in the pulse. To increase the capacity of the medium, the user of a PR4 read channel must increase the equalization of the pulses (slim the pulses) in order to limit the inter-symbol interference of adjacent pulses so that any pulse only affects two read signal samples. The increased equalization also enhances the noise accompanying the signal, making the detection task more difficult and errors more likely. U.S. Pat. No. 4,945,538 by Patel covers a similar situation but with EPR4 signaling and an RLL(1,7) code. This improves the allowed amount of inter-symbol interference, increasing it to three non-zero samples of ( . . . , 0, 1/2, 1, 1/2, 0, . . . ). Both of these techniques will allow an increase in capacity but are limited in the variety of pulse shapes which can be detected and therefore limited by how much equalization (pulse slimming) may be performed before the effect of equalizing the noise (noise enhancement) becomes intolerable.
Thus, there is a need for a flexible read channel which can accommodate a wide variety of pulse shapes as will be seen in each zone. There is also a need to allow larger amounts of controlled inter-symbol interference between pulses (pulses with more than two or three non-zero pulses) in order to continue increasing the capacity of the recording medium.